This invention relates to a time division multiple access (often abbreviated to TDMA) satellite communication network comprising a plurality of earth stations (hereinafter simply called stations) and to a station relief arrangement for use in one of the earth stations.
A conventional TDMA satellite communication network comprises a plurality of stations each of which is communicable with one another through a satellite in a time division fashion. For this purpose, each of the stations sends an up-link signal and receives a down-link signal. Each of the up-link and the down-link signals is divisible into a succession of frames each of which includes a plurality of time slots for placing data bursts.
In order to carry out favorable communication in the communication network, accurate synchronization should be established in relation to the frames and the data bursts of each of the up-link and the down-link signals. Otherwise, the data bursts sent from the stations are overlapped or superposed on the other bursts sent from the other stations when each up-link signal reaches a satellite. Overlap of the data bursts makes it impossible to carry out communication among the stations.
A preselected one of the stations is determined as a reference station for producing a succession of reference bursts which define the frames in the up-link signal sent from the reference station. The reference bursts appear through the satellite in each down-link frame and are delivered to each station. The stations establish frame synchronization with reference to the reference bursts included in the down-link signals and thereafter carry out reception and transmission.
Thus, the reference station plays an important role in making the network carry out communication among the earth stations. A fault of the reference station should therefore be avoided. Otherwise, the fault results in disruption of communication.
In U.S. Pat. No. 3,838,221 issued to W. G. Schmidt et al and INTELSAT Specification BG-1-18E (Rev. 2) Mar. 20, 1974, a time division multiple access system is disclosed which comprises a primary reference station and a subsidiary reference station. The primary reference station is operable to produce a succession of primary reference bursts in a manner similar to the reference station mentioned above. In this system, the subsidiary reference station serves to produce a succession of subsidiary reference bursts in the absence of the primary reference bursts. The subsidiary reference bursts are located at time instants at which the primary reference bursts are to be placed. Thus, the subsidiary reference station comprises a station relief circuit for taking over operation of the primary reference station to give relief thereto. At any rate, it is possible for the above-mentioned system to establish the frame synchronization in each station even on occurrence of an faults in the primary reference station.
It is mentioned here that the reference bursts sent from the reference station appear in the down-link after about 0.3 second because of large distance between stations and a satellite. It therefore takes a long time of, for example, several seconds, until the subsidiary reference station detects absence of the primary reference bursts and thereafter produces the subsidiary reference bursts. Under the circumstances, each of the stations must inevitably be operated without the primary and the subsidiary reference bursts before start of production of the subsidiary reference bursts after the primary reference bursts disappear.
As suggested before, the data bursts should be allotted to the time slots of each frame which are determined for the stations, respectively. In other words, phases of the data bursts must also be precisely controlled in each frame to assign the data bursts to the time slots, respectively. Synchronization for determining the phases of the data bursts will be called burst synchronization.
For this purpose, a synchronization signal is generally included in a data burst sent from each station and is returned as a received synchronization signal back to the same station. A received time instant of the received synchronization signal is compared with a reception reference instant assigned to each station to calculate a difference between the received time instant and the reception reference instant. Each station controls a transmission timing of the data burst with reference to the difference so that each data burst is arranged in a predetermined one of the time slots assigned to each station.
In an article contributed by Watanabe et al to 3rd International Conference on Digital Satelite Communications held at Kyoto, Japan, in 1975, under the title of "A New TDMA System for Domestic Service and its High Speed PSK Modem," description is made as regards a method of arranging a plurality of synchronization bursts which have the same format as the primary reference burst and are delivered from the respective stations in each frame in addition to each primary reference burst and the data bursts. As will later be described with reference to one figure of the accompanying drawing, the synchronization bursts are used for burst synchronization in the respective stations. In this method, each frame is divided into a synchronization part for frame and burst synchronization and an information part for the data bursts.
A subsidiary reference station is determined like in the above-mentioned system. A subsidiary one of the synchronization bursts is produced from the subsidiary reference station and positioned in each frame together with the primary reference burst.
Let the primary reference burst from the primary reference station disappear for some reason or other in each frame. In this event, each station keeps and establishes synchronization with reference to the subsidiary synchronization burst instead of the primary reference burst.
With this method, each station can quickly be switched from reception of the primary reference burst to reception of the subsidiary synchronization burst after disappearance of the primary reference bursts, as described by Watanabe et al.
Furthermore, each receiving end of the stations can be designed so that frame synchronization can respond to both of the primary reference bursts and the subsidiary synchronization bursts because the subsidiary synchronization bursts are located at a predetermined time instant as well as the primary reference bursts.
However, this method is disadvantageous in that the information part has a reduced rate in each frame with an increase of the number of participating stations because all of the synchronization bursts are disposed in each frame. As a result, a frame availability is reduced as the number of the stations increases.
In another article No. 80-6489, contributed by K. Kohiyama et al to AIAA (American Institute of Aeronautics and Astronautics), 1980, under the title of "Demand Assigned TDMA System for Digitally Integrated Services Network," burst synchronization is carried out in each station with reference to a synchronization burst which appears at every multiframe consisting of a plurality of frames. In other words, the Kohiyama et al article teaches the fact that the synchronization burst may not be produced at every frame for burst synchronization.
No discussion is, however, made in the Kohiyama et al article as regards the station relief.
The above-mentioned description is mainly based on the premise that the up-link and the down-link signals are carried by the use of up-link and down-link carrier frequencies which are common to the stations. In this connection, the above-mentioned time division multiple access system or network may be referred to as a single frequency system.
In the Schmidt et al patent, a multiple frequency system is also described for conveying each of the up-link and the down-link signals by the use of a plurality of up-link and down-link subsidiary carrier frequencies. Each station transmits the up-link signal and receives the down-link signal by switching each of the up-link and the down-link subsidiary carrier frequencies from one to another in a time division fashion. Such switching is called hopping in the art and is controlled in accordance with a burst time plan which is invariably predetermined for the multiple frequency system.
In the multiple frequency system, consideration should be made as regards failure of a primary reference station and relief of the failure, although not described in the above-mentioned patent.
Furthermore, it is preferable that the hopping can flexibly be carried out to produce the subsidiary reference bursts in the subsidiary reference station. In addition, such flexible hopping may realize various kinds of additional services.